ARMATURE LAMINATIONS 11 



tendency to stick together in the piles in which they are arranged 

 in the annealing ovens. It is practicable to anneal some qualities 

 of plates from a temperature of 900 Cent. 



It is very difficult to find a material of good initial quality 

 that will not " age " as the result of prolonged exposure to tem- 

 peratures much above 60 Cent. A sheet of the composition set 

 forth in the following table has been found to be very satisfactory. 

 Its initial hysteresis loss is fairly low when it is suitably annealed, 

 and in a number of samples tested there has been found to be 

 practically no " ageing " as the result of prolonged exposure to a 

 temperature of 60 Cent. 



Composition of a low " ageing " sheet steel. 



Carbon . ^ . . . . 0'06 per cent. 

 Silicon . .0-01 



Phosphorus . . 0*08 



Manganese . 0'5 



Sulphur . -. . 0-03 



Equally satisfactory results have, however, been obtained with 

 materials of very different compositions. 



A specification on the following lines has been employed with 

 good results : 



'* The sheets are to be of the best quality of sheet steel, and rolled 

 to a smooth, plane surface. Each sheet is to be freed from scale by 

 pickling, or otherwise, and cleaning, to be followed by cold rolling. 



" Sheets delivered under the contract must be uniform in 

 quality, composition, and treatment, should show no detachable 

 scale, and should not be annealed after the last rolling. The 

 maximum of impurities allowable is as follows : 



" Carbon . ... 0-08 



Silicon . . . . trace, or less than 0'009 

 Phosphorus . . . . 0'06 



Manganese . . 0'40 



Sulphur ... . 0*05 



"Any sheet showing less of impurities than the foregoing will, 

 if otherwise in accordance with specifications, prove acceptable; 

 but the composition, when determined upon, must be kept to a 

 uniform standard." 



The writers, however, prefer the composition given in the 

 previous table. 



Armature cores are almost always built from laminations of 

 0*5 mm. thickness. The use of a single standard thickness is 

 justified from the standpoint of requiring a minimum supply to be 



